Three woody species of Coursetia from eastern Brazil are here classified into the Rostrata clade, Coursetia caatingicola, C. rostrata, and C. vicioides. All come from the Southern Sertaneja Depression of the caatinga, and the first of these is herein described. The antiquity of this geographically confined clade is suggested by its phylogenetic isolation within Coursetia and minimum age estimates of about 9 Ma for each of the species stem clades and about 17 Ma for the Rostrata stem. These age estimates were biased young and are associated with ITS rates of substitution of about 2–3 × 10-9 substitutions per site per year, an expected rate for woody plant lineages. Multiple DNA sequence accessions coalesce with respect to nuclear ribosomal 5.8S and internal transcribed spacer (ITS) sequences for Coursetia caatingicola and C. rostrata, and with respect to chloroplast trnD-T sequences for Coursetia caatingicola. Coalescence of conspecific nuclear DNA sequence samples combined with relatively old minimum age estimates are suggestive of the evolutionary stability of local patches of seasonally dry tropical vegetation that are rich in succulent taxa. This phylogenetic signature is more likely to be found in lineages harbored by this than other types of Neotropical vegetation.
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